1. Field of the Invention
This invention relates to heat exchange systems such as may be used in propulsion systems with particular application to gas turbine engine combustors and more particularly to apparatus and methods for avoiding thermally induced fuel deposits on surfaces of a gas turbine engine combustion system heat exchanger.
2. Description of Related Art
It is well known to design aircraft propulsion systems to use gasified fuel. One purpose is to reduce production of oxides of nitrogen, NO.sub.x, during the combustion process, a very serious design consideration. U.S. Pat. No. 3,890,088, entitled "Apparatus for Reducing Formation of Oxides of Nitrogen in Combustion Processes", by Antonio Ferri, is typical of the many ideas for reducing NO.sub.x production by first gasifying the liquid fuel in a heat exchanger. The Ferri patent discloses the gasification of liquid fuel in a heat exchange apparatus and its subsequent mixing with air prior to combustion.
It is well known that gaseous fuel can be more readily mixed with air than liquid atomized fuel, thus reducing the problem of localized stoichiometric temperatures and NO.sub.x production. It is also known to mix gaseous fuel and oxidants, such as air, in order to further reduce NO.sub.x production which in turn leads to shorter combustor lengths.
The problem encountered with such a method and apparatus is the formation of fuel deposits on the surfaces of the heat exchange apparatus due to chemical reactions. The process creates gum and coke products in the form of deposits on the hot heat exchanger surfaces which are contacted by the fuel. The problem for aircraft fuel is particularly acute in the mid-temperature range temperatures from about 550.degree. to 900.degree. F. (heat exchanger surface temperature) which generates gum (primarily polymeric) products which adhere and grow from the surface. Gums embed coke and other solids as they grow into a thick surface deposit.
There are a multitude of mechanisms and chemical reactions associated with the formation of gum and coke deposits. NASA Technical Memorandum 83420 entitled "Research on Aviation Fuel Instability" by Charles E. Baker, David A. Bittker, Stephen M. Cohen and Gary T. Seng, prepared for the AGARD Propulsion and Energetics Panel Symposium on Combustion Problems in Turbine Engines, Cesme, Turkey, Oct. 3-7, 1983 provides an explanation of these phenomena in so far as rocket and gas turbine engine designers are concerned. The authors note that fuel flow rate and residence time as an important factor influencing the rate and amount of deposit which might form under any circumstances. Residence time is the foremost issue addressed by the present invention. All chemical reactions require retention of the reactants in close proximity and time to complete the reaction or series of reactions. The formation of gum and coke deposits involve an enormous number of sequential reaction steps. Therefore, if the deposit forming reactant and intermediates remain in close proximity to the hot surface which provides the thermal energy needed to promote the chemical reaction, the residence time is said to be long and the deposits will be large. However, quickly removing the reactants and intermediates from the hot surface results in a net residence time too short to complete the deposit chemical process. The potentially troublesome reactant and intermediates simple pass over and away from the hot surface with no detrimental effect.
Gasification of liquid hydrocarbon fuel has application to many combustion systems such as marine and industrial gas turbines, scramjet, ramjets, rockets and other combustion engines. The gumming and coking problem is endemic to all of these applications and the present invention may be used for all these types of applications.